In studies on the rds mouse, cyclic GMP is not elevated as in other animal models of early onset retinal degeneration. Thus, even in the small grouping of morphologically similar early onset diseases, a good deal of biochemical heterogeneity is observed. Cyclic GMP levels in rod outer segments can be controlled by metabolic agents such as diamide. Phosphorylation in the retina is important, with rhodopsin kinase occurring in two forms. Protein tyrosine kinase activity is also present in normal human retina aned in retinoblastoma cells in culture, indicating that this activity may play an important role in retinal metabolism.